1. Technical Field of the Invention
This invention relates to an aluminum alloy product. More particularly, this invention relates to an improved aluminum alloy product having a highly reflectorized surface thereon.
2. Description of the Prior Art
Highly reflective surfaces, including both specular and semi-specular reflective surfaces, have been produced on an aluminum material utilizing various techniques including proper selection of the alloy constituents, bright rolling or mechanical polishing of the aluminum surface, and processing of the highly polished or bright rolled surface in a brightening bath which may comprise either electrobrightening or chemical brightening. The highly reflective surface so produced is then protected by anodizing the aluminum to provide a thin, transparent, protective layer of aluminum oxide on the surface as is well known to those skilled in the art.
Various attempts at improving the reflectivity of the product have been proposed through the years. One approach is to vary the type of brightener used to treat the aluminum surface prior to anodizing. Typical of such an approach is the aluminum phosphate chemical brightening bath disclosed in U.S. Pat. No. 3,530,048 which uses a combination of aluminum phosphate, nitric acid, phosphoric acid and copper sulfate. The brightened aluminum surface, according to the patentees, is then anodized in a sulfuric acid bath having a concentration of from 12 to 20 wt. % at a temperature of 70.degree. to 80.degree. F. using a current of about 10 to 15 amperes per square foot.
It is also known to vary the alloy constituents to improve the reflectivity of the aluminum surface. U.S. Pat. No. 3,720,508 discloses an aluminum alloy used in the production of a highly reflective aluminum surface which contains from 0.5 to 3% magnesium, from 0.2 to 0.5% silver, from 0.001 to 0.2% iron and from 0.001 to 0.15% silicon. It is, of course, also known that excellent reflectance may be obtained from high purity aluminum. However, the cost of such a material is prohibitive. Such materials are also more difficult to work with metallurgically as well, i.e., controlling grain size, etc. Furthermore, as will be illustrated, the production of a semi-specular or milky reflective finish requires the presence of other metals such as iron not present in high purity aluminum.
The provision of additives in the anodizing bath to attempt to improve the bright or reflective surface of aluminum is also known. For example, U.S. Pat. No. 3,671,333 provides for the addition of a natural or synthetic hydrophilic colloid to the reflective aluminum surface during anodizing of the aluminum by adding the colloid to the anodizing bath. Surface coatings produced during the anodization are alleged to be much thinner and apparently more compact than previous anodized aluminum coatings which, the patentees allege, is believed to be due to the larger molecule of the colloid forming as a colloidate on the reflective surface which apparently compacts the aluminum oxide formed. The thinner coating is then alleged to provide better reflectivity while eliminating the disadvantages of a thin normal anodized coating.
Other attempts at varying the anodization process include the use of AC anodizing using a sulfuric acid bath as shown in British Pat. No. 1,439,933. High current densities of 1 to 10 amperes per square decimeter (about 10 to 90 amperes per square foot) are proposed in U.S. Pat. No. 4,252,620 for use with a highly concentrated sulfuric acid anodizing bath containing 40 to 60% sulfuric acid and oxalic acid or nickel sulfate to produce a porcelain-like texture although no improvement in reflectivity is alleged or apparently desired by the patentee.
In our parent patent application Ser. No. 590,323 and its continuation application Ser. No. 651,912, we described and claimed a novel process for the production of aluminum reflector material having a higher total reflectance value than previously attainable. The process comprises controlling the anodizing conditions of an aluminum alloy by immersing the alloy in a DC anodizing bath containing at least 26 wt. % sulfuric acid and anodizing the sheet at a current density of at least 18 amperes per square foot at a temperature of at least 60.degree. F.
Now, we have discovered that our process can produce an even higher reflectance when it is used in combination with control of the alloy constituents in the aluminum reflector material.